Sedna, a cold red world

Astronomers announced yesterday they had found the most distant planet-like object in the solar system ever, and the biggest since Pluto was discovered in 1930.

The find has revived the controversy over which heavenly bodies get to be called planets. Because Sedna, so far, appears to be in a class of its own.

Advertisement

Spotted last November by a California Institute of Technology team at the Mount Palomar Observatory near San Diego, Sedna is a round, icy world and, curiously, the second-reddest after Mars. It is 1700 kilometres wide.

At first glance, Sedna appears to fall into a category known as Kuiper-Belt Objects, or KBOs.

This is a swarm of icy bodies on the fringe of the solar system beyond the orbit of Neptune. Pluto, the outermost planet, is considered a KBO by many astronomers.

Since 1992, hundreds of KBOs have been found, including five bigger than 1000 kilometres. Two of these - Quaoar (discovered in 2002), and 2004 DW (discovered last month) - were each, at the time of their discovery, the largest worlds in the solar system that had been found since Pluto. Technological advances in telescopes allowed these extremely faint objects to be detected.

But Sedna is not just another large KBO.

Its wildly eccentric orbit brings it to within 13 billion kilometres of the sun at its closest (near where it is now) and takes it a staggering 130 billion kilometres away at its furthest - 900 times the distance between the Earth and the sun - which means it takes 10,500 years to complete an orbit.

That's way outside the Kuiper Belt, which is roughly 5 billion to 7 billion kilometres away.

"The sun appears so small from that distance you could completely block it out with the head of a pin," said the leader of the Caltech team, Mike Brown. At such a distance, he said, Sedna's temperature would be roughly minus 240 degrees.

So is Sedna (named for the Inuit goddess of the ocean) a planet? Although it's only about 600 kilometres smaller than Pluto and may have a moon, that is unlikely.

"We think it's not reasonable to call Sedna a planet, but we think it's not reasonable to call Pluto a planet," said Dr Brown. "It's not massive enough."

But neither is Sedna a long-period comet, which take thousands or millions of years to orbit the sun. These comets are thought to originate in the Oort Cloud, which begins dozens of times further away from the sun than Sedna at its most remote.

According to Dr Brown, Sedna is an "inner Oort Cloud" object, occupying the space between the Kuiper Belt and the Oort Cloud. It was probably dragged there from the Oort Cloud proper by the gravity of a rogue star that bypassed the solar system many billions of years ago.

"The star would have passed close enough to be brighter than the full moon and it would have been visible in the daytime sky for 20,000 years," said Dr Brown, adding that comets dislodged by the rogue star would have rained down on the inner solar system and hit Earth, wiping out any life.

Co-discoverer Chad Trujillo said he was "baffled" by Sedna's red colour. "We still don't understand what is on the surface of this body. It is nothing like we would have predicted or what we can currently explain."

He also said the team was trying to establish if Sedna had a moon, as this would help explain why it was rotating so slowly.